1. Field
The present invention relates to a specific block diene elastomer, the end or ends of which are composed of a polybutadiene block, intended to form an elastomer matrix of a crosslinkable rubber composition of reduced hysteresis, to a rubber composition in which it is present, to a semi-finished article in which it is present and to a tire incorporating such a semi-finished article.
2. Description of Related Art
The reduction in the hysteresis of the mixtures is an ongoing objective of the tire industry in order to limit the consumption of petrol and to thus protect the environment. This reduction in hysteresis must, however, be carried out while keeping intact, indeed even while improving, the processability of the mixtures.
Many solutions have already been experimented with in order to achieve the objective of fall in hysteresis. In particular, the functionalization of the polymers by a functional group which interacts with the reinforcing filler has appeared as an advantageous route.
Functional groups which interact with the filler have thus been attached at the chain end at the start or end of polymerization by means of functional initiators or functionalization agents. By way of example, 4,4′-bis(diethylamino)benzophenone, also known as DEAB, or other aminated functional groups which interact with carbon black have been added at the end of polymerization, as described in the patent documents FR 2 526 030 and U.S. Pat. No. 4,848,511. The polymers coupled by or star-branched by tin comprise functional groups capable of interacting with carbon black which are introduced at the end of polymerization. Mention may be made, by way of example, of the European patent document EP 0 709 235. Functional groups which interact with silica have also been attached at the chain end at the end of polymerization, such as functional groups comprising a silanol group which are disclosed in the patent document FR 2 740 778 or functional groups comprising alkoxysilane or aryloxysilane groups which are described in the document U.S. Pat. No. 5,066,721. The majority of these solutions, both for the black and for the silica, genuinely result in a limitation on hysteresis of the corresponding compositions but concomitantly in a greater difficulty in processing these same compositions.
Functional groups which interact with the filler have also been added in the middle of the chain at the end of polymerization. Mention may be made, by way of example, of patent document FR 2 930 554, which discloses aminoalkoxysilane functional groups located in the middle of the chain. This solution results in improved hysteresis and mechanical properties of the compositions in the vulcanized state and in improved raw processing of these same compositions.
Patent EP 1 278 789 describes a copolymer comprising n blocks (n=2 or 3) intended to form an elastomer matrix of a crosslinkable rubber composition, each of the said blocks comprising an essentially unsaturated diene elastomer and one or each of the said blocks forming a chain end of the said copolymer being composed of a polyisoprene. The number-average molecular weight of the polyisoprene block is between 2 500 and 20 000 g/mol and the number-average molecular weight of the block of the copolymer which is other than the said polyisoprene block is substantially between 80 000 g/mol and 350 000 g/mol. The use of the said block copolymer makes it possible to significantly optimize the results of reduction in hysteresis and processability for the said rubber composition in which it is present.